fix getdeps fetch on python 3.6
fix getdeps fetch on python 3.6

Summary: The getdeps fetch step runs early on system python, so it has to work with it.  Subprocess capture_output was from 3.7 only, so let's switch to the alternate form that also works on ubuntu 18.04's python 3.6 install

Reviewed By: Croohand

Differential Revision: D32836861

fbshipit-source-id: bd492c0ee90edadbc9124d5820ddfe515fa46f43

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